Solar Systems Installed in Harsh Climatic Conditions Require a New Set of Testing Protocols

The development of photovoltaics in regions around the world with harsh climatic conditions needs to ensure there are appropriate standards in place to provide the long-term quality, reliability and durability of solar panels and systems.

Focusing only on lowering the system’s cost per Watt, can reduce the system’s expected power output over its actual lifetime and therefore increase the levelized cost of energy (LCOE).  Consequently this can impact the overall return on investment of PV projects.

The backsheet material, for example, should protect the panel from the elements and provide electrical insulation for the 25 year expected lifetime of the panel, but not all backsheet materials will withstand higher exposure to UV and thermal stress of hot and arid climatic conditions. For the solar energy industry to continue to grow, it is important for solar investors to understand what impact materials selection has and which materials will help ensure reliable panel performance.

DuPont is the world’s leading supplier of specialty photovoltaic materials, including films for backsheets with more than 30 years of proven performance in the field and photovoltaic metallization pastes which have helped enable solar cell improvements that have yielded 30% higher cell efficiencies in the past 12 years. The company has long been an advocate of risk mitigation strategies and has been advising system owners and financiers on best practices to help improve their return on investment and lower the cost of solar electricity.

The backbone of this effort is a global and extensive field testing program that examines panels in various stages of use and in different climatic conditions to identify causes of premature aging. The program has surveyed more than 71 installations in a range of climates from hot and arid to temperate in North America, Europe & Asia Pacific, totaling more than 200MW of installed power and almost a million solar panels from 45 different manufacturers (see figure 1).

FIGURE 1 - Click to view larger version
FIGURE 2 - Click to view larger version
FIGURE 3 - Click to view larger version

The findings show that 41% of inspected modules exhibited some kind of visual defect, were less than five years’ old, with 24% of the defects affecting the cells (snail trails and busbar corrosion) and 9% being backsheet-related with defects ranging from frontside and rearside backsheet yellowing to backsheet delamination and cracking, the latter being deemed a “particularly serious defect” by Germany’s TÜV Rheinland, one the world’s leading providers of safety and performance testing, and market certification for the photovoltaic industry.  In contrast, none of the surveyed panels made with DuPont™ Tedlar® polyvinyl fluoride (PVF) film-based backsheet showed any signs of premature degradation. (See figure 2).  These findings indicate that materials choices are essential to how well and how reliably panels perform over time.

This is concerning.  Current quality standards are designed to detect early failures of solar panels, not their long-term performance or that of the materials in real outdoor conditions.   Today’s IEC testing protocols involve a single stress test at a time and do not adequately address the durability of materials to UV exposure, weathering and the synergistic effects of multiple stress factors. For instance, polyethylene terephthalate (PET) and polyvinylidene fluoride (PVDF) based backsheets would successfully meet the IEC standards.  But according to our field inspection program, we found that about 24% of the panels protected by PET and 11% by PVDF displayed some signs of visual degradation after less than 5 years in operation.   Backsheet failure caused by cracking and delamination represents a serious electrical safety hazard where panels need to be replaced.  Such replacements have a significant impact on the financial returns of solar systems, as the panels account for approximately 40% of the total system costs.

We believe the solution is the broad adoption of Module Accelerated Sequential Testing (MAST) which is the better predictor of long-term reliability, simulating real-world conditions by repeating multiple field-aging stresses (UV, heat, humidity, thermal cycling), as illustrated in figure 3.

DuPont provides recommendations on industry-standard bill of materials, and provides panel manufacturers with materials technology that can best match power output and expected lifetime goals of solar installations.

Reducing the effective system lifetime by 5 years can increase the LCOE by over 20%.  It is particularly critical to select the most robust materials under combined thermal and UV stress factors, a key recommendation to help mitigate financials risks of PV projects developed in demanding climates around the world.